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Thin ice opens lead for life on Europa

By David L Chandler

The chances that life exists on Jupiter’s moon Europa look better than ever. Researchers think the moon’s icy crust may be just a few kilometres thick – perhaps thin enough to crack open under tidal stresses and allow life in the oceans below to flourish by absorbing the Sun’s energy.

If Europa’s icy crust is thin enough, cracks would provide a habitat where life could thrive

Astrobiologists fear that if the crust is too thick, any possible life would be limited to organisms that use chemical energy from hydrothermal vents, a very restricted niche.

Now a team from the University of Arizona’s Lunar and Planetary Laboratory, led by Richard Greenberg, have been studying data from NASA’s Galileo probe, which is in orbit around Jupiter. They have built up a strong case over the past few years that Europa’s ice is less than 10 kilometres thick. And they outlined their most recent results in two talks at the Division for Planetary Sciences meeting in Birmingham, Alabama, US, as well as in Reviews of Geophysics (vol 40, p 1).

Looking at the 10 per cent of Europa’s surface that Galileo has imaged at high resolution, the Arizona group found that roughly half of it consists of “tectonic” terrain, dominated by long, straight ridges and fault lines. The other half appears to be blocks of ice randomly frozen in place, as if they had tumbled together while the surrounding medium was liquid. The researchers say both types of landscape show that cracks or molten regions in the ice have repeatedly exposed the ocean below to the surface.

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Convergence zone

Another sign that the ice must be thin is a new analysis of cracking patterns that reveals a telltale feature of Europa’s tectonic cycle. Researchers have clearly identified zones where the plates are moving apart, but what happens where the plates converge?

On Earth, colliding plates either subduct – one plate slipping below the other – or wrinkle up to form mountain ranges. Greenberg’s group has now identified convergence zones on Europa, but nothing as dramatic as on Earth. “The reason you don’t get anything like the Himalayas or subduction,” says Greenberg, “is because there’s just not much solid stuff there.”

The group’s calculations suggest that daily tides would push warmer water up into the cracks, keeping them relatively stable. And that would give any organisms access to sunlight and organic material from impacting comets, providing a habitat where life could thrive (see graphic).

The researchers imagine photosynthetic organisms might anchor themselves to the sides of cracks, to reach sunlight, while jellyfish-like creatures might ride up and down with the current.

Tidal stress

Large cracks could last for a few thousand years, the researchers say, but as Europa rotates relative to Jupiter, tidal stress points in the crust would change, and cracks would periodically freeze shut. Organisms would either need to hibernate in the ice until the crack reopened or migrate elsewhere.

But not everyone is convinced that Europa’s ice is so fragile. Robert Pappalardo of the University of Colorado in Boulder says Greenberg has done some brilliant work in demonstrating the existence of a liquid ocean, but does not agree that the ice is thin. According to Pappalardo, Greenberg’s results can be explained by flows of warmer ice like those found in glaciers on Earth.

Galileo last photographed Europa in 1999, but the ongoing disagreement means it is urgent that we return and settle the question, says astrobiologist Bruce Jakosky of the University of Colorado. “After the Earth, the two most habitable places in the Solar System are Mars and Europa. We have a vigorous Mars programme, but we don’t have a vigorous Europa programme right now.”

Hopefully that could soon change&colon; the US Congress passed a resolution last week instructing NASA to spend &dollar;40 million on just such a mission to Europa.